The performance characteristics of a given turbocharged internal combustion engine (ICE) rely on the proper selection and integration of turbomachinery elements. Component efficiency levels and flow capacities, coupled with the limits of stable compressor operation, heavily influence the overall power output, transient response, and emissions control strategies available to engine designers. To address tightening emissions requirements, most of the emerging clean combustion strategies rely on heavy exhaust gas recirculation (EGR) and subsequently place higher demands on the turbocharger and proper turbocharger-to-ICE matching.
In general terms regarding compressor performance requirements, the approach for reduced emissions via higher EGR utilization is in direct opposition to achieving a rated engine power condition. Since a portion of the charge air is recycled at high pressure, high EGR has the effect of increasing the required compressor pressure ratio, while reducing the necessary mass flow rate, thus driving the compressor towards or past the stable operating limit. In opposition to the low-flow requirements of the high EGR, low-emissions strategy is the rated power condition, typically requiring substantially higher flow with equivalent or slightly increased pressure ratio.
As the operating range requirements are increased for a single stage centrifugal compressor, an apparent limit in usable range is reached. Useable range is typically defined as a region where the compressor efficiency exceeds a desired minimum threshold value. Typically, the minimum level of efficiency is based on the duty cycle and life requirements of the various interacting system components, including the compression stage. Stable operating range can be increased further without improving the usable operating range, thus yielding minimal benefits. Due to the extremely broad required range, oftentimes the solution is two compressors (small and large) designed to deliver high performance levels at differing flow rates. The changeover between the small and large compressors is regulated via valves. To achieve the desired range in a single compressor requires compromises in the design process. Traditionally, recirculating casing treatments are applied to the baseline compressor to extend the stable operating range. The primary approach is that flow is removed from the mid-passage region (between compressor leading and trailing edge) and then re-introduced upstream of the compressor inlet.
To improve internal combustion engine emissions levels, exhaust gas is recirculated in varying amounts. Typically, exhaust gas is removed upstream of the turbine at high temperature and pressure and re-introduced at high pressure prior to entering the cylinders of the engine. The EGR loop can pass through an EGR cooler and can have a throttle valve in place to modulate the flow rate.